1. Field of the Disclosure
This disclosure generally relates to a method and apparatus for imaging of earth formations using resistivity measurements in a borehole that has a conducting fluid.
2. Background of the Art
Electrical logging devices are often used for imaging boreholes. There are two categories of electrical logging devices that are typically used for logging wellbores for providing borehole images. The first category relates to galvanic devices, wherein a measure electrode (current source or sink) is used in conjunction with a return electrode. In one such device, current flows from a current source to the measure electrode, which current is induced into the formation surrounding the borehole. The current from the formation returns back to the current source via the return electrode. The second category relates to inductive measuring tools in which an antenna within the tool induces a current flow within the earth formation. The magnitude and/or phase of the induced current is detected using either the same antenna or a separate receiver antenna.
There are several modes of operation of a galvanic device. In one mode, the current at a current electrode is maintained constant and a voltage is measured between a pair of monitor electrodes. In another mode the voltage of the measure electrode is maintained constant and the current flowing from the measure electrode is measured.
When measurements are made using resistivity tools in a borehole that has a conductive fluid, undesired current flowing axially along the borehole fluid shunts the desired current path through the formation. To mitigate this effect, a common practice is to attempt to maintain the borehole fluid at the same potential as the measure electrode. The present disclosure provides an improved resistivity tool and method for borehole imaging.